1. Technical Field
The present invention relates to a process for oligomerizing alpha-methylstyrene and an initiator therefor. More particularly, the invention is directed to preparing oligomers of alpha-methylstyrene having a number-average molecular weight (Mn) of about 5,000 and below.
2. Background
Alpha-methylstyrene (xcex1-methylstyrene) has been utilized as a monomer and comonomer in many polymerization processes. The polymerization products of such processes have been put to use as adjuvants in polymeric compositions, improving the impact and heat-resistant properties of polymers. When admixed with polyvinyl chloride (PVC) low molecular weight poly(xcex1-methylstyrene) reduces the fusion time and melt viscosity of the PVC composition and improves its heat stability. Low molecular weight xcex1-methylstyrene oligomers also improve the melt fracture and shear burning resistance of PVC at high shear processing rates (Wilson A. P.; Raimondi, V. V.; Polym. Eng. Sci. (1978). 18(11), 887-92).
Oligomers of xcex1-methylstyrene have also been employed as processing aids for chlorinated polyvinyl chloride (CPVC). Incorporation of poly(xcex1-methylstyrene) into CPVC reduces fusion time and melt viscosity and improves fusion, melt flow, and stability, without deleteriously affecting the desirable properties of the polymer composition (Raimondi, V.; Wilson, Alfred P.; Soc. Plast. Eng. Tech. Pap. (1978), 24, 747-9).
Heretofore, low molecular weight poly(xcex1-methylstyrene) has been produced by polymerizing xcex1-methylstyrene in the presence of Lewis acid initiators such as BF3, BCl3, and SbCl5 in combination with an aluminum halides and the like. In commercial practice, low molecular weight poly(xcex1-methylstyrene) has been provided by polymerizing xcex1-methylstyrene in the presence of a BF3/water mixture. However, such catalyst systems have been demonstrated to be highly corrosive, resulting in many engineering problems leading to eventual shut down of the manufacturing plant. The corrosive nature of the BF3/water and aluminum halide mixtures have necessitated the use of expensive metal alloys in manufacturing plant design. Even when such measures have been employed, the corrosive nature of the residual catalyst system dictated that the spent catalyst system had to be neutralized and removed from the obtained product. What is desired is an oligomerization process involving mild reaction conditions and a simple initiator system.
Accordingly, it is a general object of the present invention to provide a process for the oligomerization of xcex1-methylstyrene.
It is another object of the invention to provide a process for xcex1-methylstyrene oligomerization utilizing inert reaction media.
It is yet another object of the invention to provide an initiator system that does not need to be neutralized or removed from the oligomerized product.
It is still another object of the invention to provide a poly(xcex1-methylstyrene) product having a number average molecular weight of about 5000 and below.
It is another object of the invention to provide an oligomerized xcex1-methylstyrene product having a number average molecular weight of about 500 to 5000.
In accordance with the present invention, it has been discovered that xcex1-methylstyrene can be oligomerized when contacted with a catalytic amount of a single component initiator comprising a cation and a weakly coordinating anion (WCA) at a temperature ranging from about xe2x88x9215xc2x0 C. to about 40xc2x0 C. By weakly coordinating anion is meant that the anion is only weakly coordinated to the cation complex. The anion is sufficiently labile to be displaced by monomer. The WCA functions as a stabilizing anion to the cation complex and does not transfer to the cation complex to form a neutral product. The WCA is relatively inert in that it is non-oxidative, non-reducing, and non-nucleophilic.
The cation portion of the single component initiator useful in the process of the invention is selected from lithium or a carbocation of the formula:
C+(R1)(R2)(R3)
wherein R1, R2, and R3, independently represent hydrocarbyl and substituted hydrocarbyl radicals.
The WCA portion of the single component initiator useful in the process of the present invention is selected from a borate of the formula:
B(R4)3(R5)
wherein R4, independently represents a fluorinated aryl radical and R5 represents a radical selected from hydrocarbyl, fluorinated hydrocarbyl or fluorinated aryl.
International Published Patent Application No. WO 95/29940 discloses a cationic catalyst system for polymerizing olefinic and styrenic monomers to high polymers having molecular weights above 10,000 Mn, and most preferably above 100,000 Mn. The catalyst system includes a cationic component selected from a hydrocarbyl substituted carbocation or a cyclopentadienyl transition metal cations and a non-coordinating anionic complex including hydrocarbyl substituted borates. The catalyst system is combined with the monomer in slurry or solution and the polymerization reaction is conducted at temperatures below about 20xc2x0 C., and more preferably between xe2x88x92150 and xe2x88x9220xc2x0. There is no disclosure of a process for making oligomers of xcex1-methylstyrene.
T. D. Shaffer and J. R. Ashbaugh, J. Poly. Sci., Part A, Vol. 35, 329-344 (1997) (Table IX), have reported polymerizing (xcex1-methylstyrene) to a Mn of 6400 in the presence of a multicomponent catalyst system consisting of lithium n-butyltrispentafluorophenylboron and the initiator 1,3-bis(1-chloro-1-methylethyl)-5-tert-butylbenzene. However, there is no disclosure of a single component catalyst system capable of producing poly(xcex1-methylstyrene) oligomers having molecular weights of 5000 or below (Mn)
The present invention is directed to a non-corrosive initiator system and process for oligomerizing (xcex1-methylstyrene) to low molecular weight poly(xcex1-methylstyrene). By low molecular weight is meant that the xcex1-methylstyrene oligomer has a number average molecular weight (Mn) of 5000 and below (relative to a polystyrene standard). In one aspect of the invention the molecular weight of the oligomerized xcex1-methylstyrene ranges between 500 and 4500 Mn. In another aspect of the invention the molecular weight of the oligomerized xcex1-methylstyrene ranges between 1000 and 4000 Mn. The desired oligomers are made in inert (non-corrosive) media in the absence of water and the corrosive compounds of the prior art, obviating the need for the use of expensive alloys in the physical plant and by-passing the necessity to neutralize and remove catalyst components from the resulting product.
In one embodiment of the invention the polydispersity (Mw/Mn) is 10 or less. In another embodiment the polydispersity is 5 or less. In still another embodiment it ranges from 1.5 to 4, and in another embodiment it ranges from 2 to 3.5.
The single component catalyst system of the invention is represented by the formula:
[M+][WCAxe2x88x92]
In the formula above, M represents lithium or a carbocation of the formula: 
wherein R1, R2, and R3, independently represent hydrocarbyl and substituted hydrocarbyl radicals. In one aspect of the invention, the hydrocarbyl and substituted hydrocarbyl radicals are independently selected from hydrogen, linear or branched (C1 to C20) alkyl, (C5 to C10) cycloalkyl, (C6 to C14) aryl, and (C7 to C24) aralkyl, provided that only one of R1, R2, and R3 can be hydrogen at any one time. By substituted is meant that the hydrocarbyl radical can be substituted with a halide selected from chlorine, fluorine, bromine, and iodine, or with another hydrocarbyl group selected from (C1 to C10) alkyl, (C5 to C10) cycloalkyl, (C6 to C14) aryl, and (C7 to C24) aralkyl. In another aspect, the carbocations of the invention contain aryl radicals wherein R1, R2, and R3 are selected from phenyl, tolyl, xylyl, and biphenyl. In still another aspect, the carbocation is selected from triphenylcarbenium or trityl.
The WCA component of the initiator complex represented in the above formula is a boron containing compound of the formula:
B(R4)3(R5)
wherein R4 independently represents a substituted (C6 to C14) aryl radical and wherein two or more of the available valences on the aryl radical are substituted by fluorine, linear and branched (C1 to C20) fluoroalkyl, fluorophenyl and combinations thereof. Other substituents on the substituted aryl radical can include linear and branched (C1 to C10) alkyl, linear or branched (C2 to C20) alkenyl, (C5 to C10) cycloalkyl, (C6 to C14) aryl, and (C7 to C24) aralkyl. By fluoroalkyl is meant that at least one hydrogen atom on the alkyl radical is replaced by a fluorine atom. By fluorophenyl is meant that at least one hydrogen atom on the phenyl radical is replaced by a fluorine atom. The degree of fluorination of the fluoroalkyl and fluorophenyl groups can range from one hydrogen atom on the alkyl and phenyl groups being replaced by a fluorine atom (e.g., monofluoromethyl, monofluorophenyl) to full fluorination (perfluorination) wherein all available hydrogen atoms on the alkyl and phenyl groups have been replaced with fluorine atoms (e.g., trifluoromethyl (perfluoromethyl), and pentafluorophenyl (perfluorophenyl)). R5 represents R4 as defined above or a hydrocarbyl radical including hydrogen, linear or branched (C1 to C20) alkyl, linear or branched (C2 to C20) alkenyl, (C5 to C10) cycloalkyl, (C6 to C14) aryl, and (C7 to C24) aralkyl.
Representative borate anions include
tetrakis(pentafluorophenyl)borate,
tetrakis(3,5-bis(trifluoromethyl)phenylborate,
tetrakis(3,5-difluorophenyl)borate,
tetrakis(2,3,4,5-tetrafluorophenyl)borate,
tetrakis(3,4,5,6-tetrafluorophenyl)borate,
tetrakis(3,4,5-trifluorophenyl)borate, methyltris(perfluorophenyl)borate,
ethyltris(perfluorophenyl)borate, phenyltris(perfluorophenyl)borate, and
tetrakis(perfluorobiphenyl)borate.
Suitable initiators of the invention include lithium
tetrakis(perfluorophenyl)borate and trityl tetrakis(perfluorophenyl)borate.
The initiators of the present invention can be combined with the (xcex1-methylstyrene) monomer as a pre-formed single component initiator of the formula:
[M+][WCAxe2x88x92]
wherein M and WCA are as previously defined or they can be formed by combining the cation and anion precursor compounds in monomer wherein the active initiator is formed in situ.
In one aspect of the invention, the ratio of monomer to initiator employed in the present process can range from between about 2000:1 to about 1,000,000:1 (mole:mole basis). In another aspect, the ratio of monomer to initiator can range from about 10,000:1 to about 20,000:1 (mole:mole basis).
The process comprises contacting xcex1-methylstyrene with a catalytic amount of the initiator defined above at a temperature range of about xe2x88x9215xc2x0 C. to about 35xc2x0 C. In another aspect the reaction temperature can range from about 0xc2x0 C. to 30xc2x0 C., and in a further aspect between 5xc2x0 C. and 25xc2x0 C. Surprisingly, it has been discovered that the molecular weight of the xcex1-methylstyrene oligomer can be kept consistently below about 5000 Mn by running the reaction at a temperature between xe2x88x9215xc2x0 C. and 35xc2x0 C.
The reaction can be conducted in bulk, slurry, or in solution. Suitable diluents include hydrocarbons and aromatic hydrocarbons. Suitable hydrocarbon diluents can be selected from linear and branched C3 to C6 alkanes such as propane, butane, pentane, hexane and cyclohexane. Suitable aromatic diluents include benzene, toluene, xylenes, and cumene.
The hydrocarbon diluents are non-solvents for the oligomeric product. Accordingly, the xcex1-methylstyrene oligomeric product will precipitate from solution following the reaction, allowing for the easy recovery of the product via conventional separation means such as filtration. When the oligomerization reaction is conducted in an aromatic diluent, the product is a cement comprising the xcex1-methylstyrene oligomer and diluent. The diluent can be removed by extruding the cement composition on a conventional devolatizing extruder.
In solution, the weight percent of monomer in diluent preferably ranges from 10 to 80 percent, more preferably 20 to 70 percent, and more preferably 30 to 60 percent.
The oligomers produced in accordance with this invention can be used for many purposes well known in the art, for example, as chemical intermediates, and as processing and heat stability aids for PVC and CPVC.
The following examples are presented solely for illustrative purposes and serve to exemplify various aspects of the invention. They not intended as a restriction on the scope of the appended claims.